He explained that some people also have "white coat" hypertension, meaning their blood pressure spikes when at the doctor's office. With 24-hour monitoring, they said they found 30 percent of their patients had "white coat" hypertension.

Ingham believes finding the average of multiple blood pressure measurements during a 24-hour period is a much better predictor of heart attack and stroke risk and said it has changed some patients' lives.

"About 10 percent of people we've done this on have either stopped the medication or decreased a dose," Ingham said.

Cummings also found that some patients who thought they did not have high blood pressure actually did.

"We found about 20 percent of our patients had masked hypertension," he said.

Ingham said a normal daytime blood pressure average is less than 135 over 85 and a normal nighttime average is less than 120 over 75.

"We really target that sleep blood pressure as the most important component," said Ingham who believes it's a better predictor of heart attack and stroke risk. "It's not just being asleep that's important. Your position, being supine, laying flat is important... the way your heart beats the way your blood vessels constrict and things like that."

They say giving a patient a cuff to wear for 24 hours is not expensive. Still, not all experts believe that 24-hour monitoring is necessary. Cummings and Ingham hope to prove it's a much better option.